Potentiometers



April 1960 M. B. COTTON 2,934,731

' POTENTIOMETERS Filed Jan. 51, 1951' s Sheets-Shea 1 INVENTOR M IFHAEL BEN COTTON April 26, 1960 M. B. COTTON 2,934,731

POTENTIOMETERS Filed Jan. 31, 195'? -3 Sheets-She et 2 INVENTOR YMICHAEL EN COTTON TTORNEY April 26, 1960 M. B. COTTON 2,934,731

POTENTIOMETERS INVENTOI MICHAEL BEN COTTON 3 Sheeifi' 3 3B nnuumnnm,

Filed Jan. 31, 1957 United States Patent 2,934,151 POTENTIOMETERS Michael Ben Cotton, East Molesey, England, assignor to The Sperry Gyroscope Company Limited, Brentford, England, a British company Application January 31, 1957, Serial No. 637,482

Claims priority, application Great Britain February 13, 1956 5 Claims. (Cl. 338-128) This invention relates to variable impedance units of the kind in which the impedance connected between two terminals depends upon the setting of an adjustable tap of a distributed impedance.

One object of the present invention is to provide such a unit with greater sensitivity in terms of change in impedance for a given tap movement.

In accordance with the present invention a variable impedance unit includes at least two distributed impedances and an adjustable tap for each impedance, the taps being ganged to move simultaneously in relation to their respective impedances to vary the proportion of each which is connected between two terminals of the unit.

The duplication of the distributed impedances combined with the gauging of the taps simulates the effect of gearing, for the actual impedance change due to the two distributed impedances will be double the change that would be a result of the same movement of a tap on a single distributed impedance.

Preferably the impedance unit includes two, or a multiple of two, distributed impedances of which the taps are electrically connected in pairs.

In this way each connected pair of taps serves to connect the two associated impedances, or parts of them, in series between the said two terminals of the unit, while the limitation of the number of impedances to a multiple of two will enable the output terminals to be both connected to the distributed impedances (the preferred case), or both connected to the taps, and then the terminals can be connected to a stationary part of the unit so that there will be no necessity for slip rings or other moving output connections. Usually, of course, it will be arranged that in consequence of adjustment of the taps of any one pair of impedances, the proportions of the two impedances which are connected between the two terminals of the unit, both change in the same sense.

In one arrangement in which the effect of a small deflection is multiplied fourfold, four distributed impedances in two pairs have their taps ganged to move simultaneously in relation to their respective impedances to increase the proportions of the impedances of one pair which are connected between one pair of terminals of the unit, and to decrease the proportions of the impedances of the other pair which are connected between another pair of terminals (which may have one terminal in common with the first pair). The two pairs of terminals can be connected in opposition each across one half of a centretapped battery to produce a current through a common load which will be varied by four times the amount it would have been varied if a single distributed impedance had been used. c

Preferably the unit is a rotary unit, and the terminals are on the stator, so that no slip rings are necessary. In general it would be more convenient to have the taps on the rotor and the distributed impedances on the stator. In such a rotary arrangement the different distributed imiCC pedances may conveniently be wound on different of a single annular former.

The invention has special application as applied to a pickolf element for a sensitive instrument such as a gyroscope. Thus the motor may be arranged to be driven by a movable element of a sensitive element, for example, a gimbal ring of a gyroscope, so that consequent adjustment of the taps serves as a measure of the angular deflection of the gimbal ring or other movable element. The use of the multiple distributed impedances reduces the torque opposing deflection of the movable element, which is a great advantage where the element is a gyroscope ring.

In one application of the impedance unit as used to produce an output voltage dependent on the angular defiection of a gimbal ring of a gyroscope, means responsive to the voltage are used to operate a control surface of an aircraft in the sense to oppose the change which introduces the deflection of the gyroscope ring, while a feedback path may be arranged to balance a signal dependent upon the deflection of the control surface against the output voltage.

Thus, the unit is capable of being used as a pick-off device in an automatic control system for aircraft for detecting, for example, heading deviation of the aircraft from a preset heading to be maintained by the automatic control system.

During automatic control, yaw angles are quite small, generally of the order of 1 on either side of the heading to be maintained, and practically never more than 3 on either side. It is sufficient, therefore, for a pick-otf to operate over angles of a few degrees on either side of a zero position. The fact that normal control is substantially within 1 of the zero position makes it difficult with ordinary variable impedances to obtain an output signal with sufiicient sensitivity.

However the unit embodying the invention is more sensitive.

The invention may be carried into practice in different ways, and one specific embodiment will now be described by way of example with reference to the accompanying drawings in which:

Figure l is a schematic illustration of one form of potentiometer embodying the invention;

Figure 2 is a schematic illustration of apparatus in an aircraft embodying the potentiometer shown in Figure 1;

Figure 3 is a sectional elevation of a practical form of the potentiometer illustrated in Figure 1;

Figure 4 is a view on the line IV-IV in Figure 3 The rotary potentiometer shown comprises a first of resistors 1, 2 having an associated pair of wiper-arms v 3, 4 rigidly connected together and to a pair of wiperarms 7, 8 of a second pair of resistors 5, 6. The two wiper-arms of each pair are electrically connected together and the four arms are fixed in angularly-spaced positions around the axis of a rotatable shaft 12.

The three terminals of the potentiometer are indicated by reference numerals 9, 10 and 11, the terminal 9 being common to the clockwise end of the resistor 1 and to the counter-clockwise end of the resistor 5. The terminal 10 is connected to the clockwise end of the resistor 2 and the terminal 11 to the counter-clockwise end of the resistor 6 as shown. As will be seen, the arrangement is such that the wiper arms 3 and 4 connect the terminals 9 and 10 in series through portions of the resistors 1 and 2, which portions are increased or decreased in consequence of rotation of the wiper-arms 3 and 4. The

wiper arms 7 and 8 connect the terminals 11 and 9 in series through portions of the resistors and 6, which portions are also increased or decreased in consequence of rotation of the wiper-arms 7, 8. Rotation of the shaft 12 produces rotation of the wiper-arms 3, 4, 7 and 8 simultaneously by amounts depending on the extent of rotation of the device 12. If the rotation is such that the length of the resistors connected between the terminals 10 and 9 is increased, the length of the resistors connected between the terminals 11 and 9 is decreased.

In Figure 2, the potentiometer illustrated in Figure l is shown as connected in a circuit with a battery 13 and a. voltage utilisation device 17. The terminals 18 and 11 are respectively connected to the positive and negative terminals of the battery 13 across which terminals is also connected a potentiometer 14, whose centre-point 15 is connected to the centre-point 16 of the battery 13. The wiper-arm 19 of the potentiometer 14 is connected to the terminal 9 through an amplifier 18 whose output is delivered to the voltage utilisation device 17.

The whole circuit constitutes a bridge providing an output voltage between the terminal 9 and the wiperarm 19 which has a sense and magnitude proportional to the sense and magnitude of angular displacement of the shaft 12 from a null position in which the bridge is balanced. The balance of the bridge can be adjusted by adjusting the position of the wiper-arm 19 of the potentiometer 14 in relation to its centre-point 15.

Figures 3, 4 and 5 illustrate a practical construction of the potentiometer unit described with reference to Figures 1 and 2.

The unit comprises a body 21 within which is secured an annular former 22 of flat cross section on different circumferential parts of which are wound the resistors 1, 2, 5, and 6. A wiper arm assembly 23 carrying the wiper arms 3, 4, 7 and 8 is both rotatably and slidably mounted on the shaft 12 which extends through the body 21 and is rotatably supported in the body at each end in ball bearings 24 and 25.

The wiper arm assembly 23 is normally held with the wiper arms out of engagement with the resistors 1, 2, 5 and 6 by means of a pair of radial springs 27 and 28 shown in Figure 5, and is thus normally in the position shown in Figure 3. The assembly 23 can, however, be clutched to the shaft 12 by means of an electromagnetic clutch 29 comprising a clutch plate 31 of ferro-magnetic material keyed to the shaft 12 by a key 32 and a cooperating member constituted by the end faces of a pot magnet 33 having an exciting winding 34. The shaft 12 extends through a bore in the central pole piece of the pot magnet so that the pot magnet can slide both along and around the shaft 12. The wiper arm assembly is secured to the pot magnet. Excitation of the winding 34 causes the pot magnet 33 to move axially until it engages and is clutched to the clutch plate 31 against the restraint of the springs 27 and 28. Once the clutch is engaged the wiper arm assembly 23, which moves with the pot magnet, is no longer free to move in relation to the shaft 12, but can rotate with the shaft 12 with its wiper arms 3, 4, 7, and 8 making contact with different points on the resistors 1, 2, 5 and 6 as it does so.

The resistors 1, 2, 5 and 6 are wound as shown in Figure 4 on successive circumferential portions of the former 22. Each resistor extends throughout about 60 of the circumference of the former and consists of a central portion 36 which subtends an angle of about at the centre of the unit and a portion 37 on either side of similar size, of which the turns are connected together by a strap 38. In the balance position of the potentiometer each of the wiper arms 3, 4, 7 and 8 is in the centre of the central portion 36 of its corresponding resistor 1, 2, 5 or 6.

The wiper arm assembly 23 comprises an insulating sleeve 41 slidably and rotatably mounted on the shaft 12. The left hand portion of the sleeve 41 (as seen in Figure 3) extends beyond the pot magnet 33 and carries two spaced plates 42 and 43 each having holes aligned with those of the other through which pass bolts 44 for securing the whole assembly 23 to the face of the pot magnet 33 remote from the clutch plate 31. Between the two plates 42 and 43, and surrounding the sleeve 41, are a number of spaced insulating washers 45. Between the first two adjacent pairs of washers 45 from the left are spring attachment plates 46 which also carry the current to the exciting winding 34 of the clutch 29. Between the next adjacent pair of washers is a composite wiper arm constituting the two wiper arms 3 and 4, while between the last washer 45 and the plate 43 (which is of insulating material) is a composite arm constituting the wiper arms 7 and 8. The wiper arms 7 and 8 are connected together but are insulated from the wiper arms 3 and 4 which are themselves connected together.

External connections 9, 10 and 11 from the resistors 1, 2, 5 and 6 are taken out through one end of the body 21. One only, the connection 9, is shown in Figure 3.

A plate 47 (Figure 5) bolted to the body 21 has its two ends bent to extend radially inwardly to provide mechanical limit stops 48 and 49 which co-operate with a lug 51 formed on the plate 42 to limit the extent of angular movement permitted to the wiper arm assembly. When these limits are reached the clutch will slip if the shaft 12 continues to rotate. In order to facilitate slipping of the clutch 29, 34, switch contacts 58 and 59 are provided for deenergizing, upon contact, the clutch winding 34. Thus, upon such contact of switch elements 58 and 59, the flow of current through winding 34 is interrupted, and during the time when this contact is made, springs 27 and 28 immediately act to move the wiper arm assembly toward its neutral position and, by so doing, automatically open contacts 58 and 59 to allow current again to flow through the winding 34. The net effect of this arrangement is that upon rotation of the shaft 12 sufficiently to cause wiper arm assembly to reach the limit position, the assembly is stepped back a short distance to allow further rotation of the shaft 12. If this shaft continues to turn, switch elements 58, 59 again make contact and the wiper arm assembly is again stepped back. The magnitude of this step-back is dependent upon the speed of operation of the cut-out and the stiffness of the centralizing springs 27, 28. The mechanical limit stops 48, 49 are so arranged as to come into effect immediately after electrical contact between switch elements 58 and 59 is made. The mechanical limit stops act to prevent damaging of the cut-out switches if the shaft 12 is being rotated very rapidly. In any case the magnitude of the voltages which can be derived by the rotation of the wiper arm assembly 23 is limited by the wiper arms coming into contact with the short-circuited side portions 37 of the various resistors, for when this occurs no further increase of voltage can be obtained.

When the clutch is disengaged, the wiper arm assembly 23 is held centralised as shown in Figure 5, by means of the springs 27 and 28, so that the moment of engagement the wiper arm assembly will always be in this position and hence the unit will be effective to detect any movement of the shaft 12 from the position it had when the clutch was engaged.

The centralising springs 27 and 28 used to centralise the rotor assembly about its axis of rotation have no deleterious effect on the operation of the apparatus at least if the angular displacements that the unit is intended to detect are small and if the shaft is power driven.

If desired, a separate spring assembly may be provided for centralising the rotor assembly about its axis.

A unit as described may be used in an automatic pilot for aircraft in which it is desired to produce a control signal corresponding to the angular displacement of the aircraft about one of its primary axes from a predetermined position, for example, the angular deviation in headingot the aircraft from a preset heading. For this purpose, as shown diagrammatically in Figure 2, the shaft 12 may be driven by a servo motor 52 energised by data received, through a selsyn transmission system, from a direction-defining instrument, such as a gyro-magnetic compass 53, concerning the angular deviation of the aircraft about its aximuth axis, and the output provided by the potentiometer will be a measure of the sense and magnitude of the angular deviation of the aircraft. The output signal from the potentiometer may be applied in a well-known manner to control a control surface 54 to correct the heading deviation. The utilisation device 17 may take the form of an electro-magnetic device such as a Laws relay, which serves to control the position of a relay valve 55, which in turn controls the supply of fluid to actuate a hydraulic servo-motor 56.

The wiperarm 19 of the potentiometer 14 may be driven by the control surface 54 so as to produce a repeat-back signal in well-known manner. As a result the displacement of the control surface will be proportional to the signal from the potentiometer forming part of the unit.

The clutch is provided in order that a predetermined angular relationship may be developed between the shaft 1'2 of the wiper-arm assembly 23 and thus between the wiper-arms 3, 4, 7 and 8 and the resistors. It is necessary for this provision to be made in order that, for example, changes in heading of the aircraft may be produced. When a change in heading is required, the clutch is deenergised and the aircraft controlled until the new heading is reached. De-energisation of the clutch will cause the wiper-arm assembly to be centralised by means of the springs to the balanced position of zero output from the potentiometer, so that the potentiometer is conditioned to provide an output voltage measuring the angular displacement of the aircraft from the heading prevailing at the time when the clutch is re-energised.

Of course for different applications, the distributed impedances may be arranged in different ways.

Thus two impedances which are to be increased together may be wound on different arcuate sections of a rotary former with their clockwise ends connected together. The unit terminals may then be connected respectively to the two taps on the stator.

Again, this arrangement could be duplicated with four impedances on the rotor, the two of one pair with their clockwise ends connected together, and the two of the other pair with their anti-clockwise ends connected to gether. The internal connections may be similar to those shown in Figure 2. Rotation will increase the portions of the two impedances which are connected between one pair of terminals, and decrease the portions of the two impedances which are connected between the other pair.

What is claimed is:

1. A variable voltage divider comprising two input terminals and an output terminal, two pairs of distributed impedances and an adjustable tap for each impedance, one pair of impedances having adjacent ends connected to said output terminal and their other ends unconnected, and the other pair of impedances each having opposed ends connected respectively to said input terminals and their other ends unconnected, means for electrically interconnecting the two taps associated with one impedance of each pair of impedances whereby portions of said one impedance of each pair of impedances are connected in series between said input and output terminals, and means mechanically ganging all of saidtaps for simultaneous movement in both direction and magnitude whereby to vary the proportions of the impedance offered by each one of each pair of impedances between said input and output terminals.

2. A potentiometer pick-oft for sensitive instruments comprising first, second, third and fourth sequentially spaced, distributed impedances, a wiper tap for each impedance, and means for supporting said impedances and taps for relative movement, said first and second impedances having their adjacent ends thereof connected with an output terminal and their opposed ends unconnected, and said third and fourth impedances having their opposite-ends respectively connected to a pair of input terminals and their adjacent ends unconnected, the taps of said first and fourth impedances being electrically interconnected and the taps of said second and third impedances being electrically interconnected whereby portions of said first and fourth impedances and said second and third impedances are connected in series respectively between each input terminal and said output terminal, and said taps being all mechanically interconnected for simultaneous movement in magnitude and sense relative to said impedances, whereby simultaneously to vary the effective portions of each of said impedances between said input and output terminals in accordance with said relative movement.

3. A potentiometer pick-off of the character set forth in claim 2, wherein said impedances are sequentially spaced about the periphery of an annular former and wherein said taps are mounted for rotation about the axis of symmetry of said annular former.

4. A potentiometer pick-off device comprising two input connections and an output connection, a plurality of two-terminal resistance element arranged in pairs and extending from each side of said output connection, one terminal of each element being fixed and the other being variably positionable on said element whereby to vary the effective resistance thereof, said variable terminals of each pair of elements being electrically connected and said variable terminals also being mechanically connected for simultaneous movement, the fixed terminal of one of said elements of each pair of elements being connected with said output connection and the other fixed terminal of each pair of elements being connected respectively with said input connection, whereby each resistance element of each pair is connected in series between said input and output connections through said movable terminals, said resistance elements being so directionally distributed with respect to said input and output connections and with respect to the movement of said movable terminals that such movement serves to vary the effective impedance of each resistance element of one pair in the same sense and simultaneously to vary the etfective impedance of each resistance element of the other pair in the opposite sense.

5. A potentiometer pick-01f device comprising an an nular former'member, a rotor member supported for rotation about the axis of symmetry of said former, a pair of input connections and an output connection, a plurality of resistance elements mounted in pairs on said former and symmetrically arranged therearound, the counterclockwise terminals of each winding of one pair being connected respectively with said output connection and said input connection, the clockwise terminals thereof being unconnected, and the clockwise end of another pair of windings being connected respectively with the other input connection and said output connection, the counterclockwise terminals thereof being unconnected, a tap for each resistance element mounted on said rotor member for simultaneous movement therewith in bothdirection and magnitude, and the two taps of each pair of windings being electrically interconnected whereby portions of each pair of windings are electrically connected in series between its respective input connection and said output connection thereby, upon relative movement between said rotor member and said former member, to vary the impedances offered by each pair of impedances between said input and output con-F nections in opposite senses by a ratio dependent upon the number of pairs of resistance elements and taps between said input and output connections.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS 8 Tiebel Mar. 11, 1941 Metcalf Mar. 25, 1941 Isserstedt Oct. 30, 1945 Frische et a1. Apr. 16, 1946 

